Fertilizer Calculator
How much fertilizer product to spread to apply a target rate of nitrogen, phosphate or potash, from the N-P-K numbers on the bag, with the other nutrients it delivers and the bags to buy.
Result…
How it's calculated
Product rate = target nutrient rate ÷ (nutrient % ÷ 100) Product (kg) = product rate (g/m²) × area (m²) ÷ 1000 Product (lb) = product rate (lb per 1,000 ft²) × area (ft²) ÷ 1000 Each nutrient applied = product rate × its % ÷ 100 · bags = product ÷ bag size, rounded up
Fertilizer labels give three numbers, N-P-K: the percentage by weight of nitrogen, phosphate and potash. To apply a target amount of one nutrient, divide the target by that percentage as a decimal: 1 lb of nitrogen per 1,000 ft² from a 26-5-10 product takes 1 ÷ 0.26 = 3.85 lb of product per 1,000 ft². Multiply by the area for the total, and by the other two percentages to see what else that product delivers. The target rate is not built in: it depends on the plants, the soil, the season and local guidance, so you enter it.
Your numbers, step by step
About the Fertilizer calculator
This fertilizer calculator works out how much product to spread to apply a target amount of a nutrient, using the three N-P-K numbers on the bag. Type the label as it appears, such as 26-5-10, choose whether your target is nitrogen, phosphate or potash, and enter the rate you want per square metre, per 100 m², per hectare, per 1,000 ft² or per acre. The product rate is the target divided by that nutrient's percentage, so 1 lb of nitrogen from a 26% product takes 3.85 lb of fertilizer. The calculator then multiplies by your area, shows what the other two nutrients add up to, and rounds up to whole bags. The target rate itself comes from you, from a soil test, local advice or the product label, because it depends on your plants, soil, season and region.
How to use it
- Pick the “Units” and enter the “Area to feed”.
- Type the “N-P-K on the label” as three numbers, such as 20-5-10.
- Choose the “Target nutrient”, then the “Target rate unit” and the “Target rate of the nutrient”.
- Enter the “Bag size” and, if you want the cost, the “Price per bag (optional)”.
- Read the fertilizer needed, the product rate to set on the spreader and the nutrients it delivers.
Worked example
A 4,000 ft² lawn, a 26-5-10 product and a target of 1 lb of nitrogen per 1,000 ft². Product rate = 1 ÷ 0.26 = 3.85 lb per 1,000 ft². Total = 3.85 × (4,000 ÷ 1,000) = 15.38 lb, so two 15 lb bags, with about 14.6 lb left. That rate also applies 3.85 × 0.05 = 0.19 lb of phosphate and 3.85 × 0.10 = 0.38 lb of potash per 1,000 ft².
Updated
Questions people ask
How do I work out how much fertilizer to apply?
Divide the nutrient rate you want by that nutrient's percentage on the label as a decimal, then multiply by the area. This is the method Penn State Extension uses: 1.0 lb of nitrogen ÷ 0.26 = 3.8 lb of a 26-5-10 product per 1,000 ft².
What do the three numbers on a fertilizer bag mean?
They are the percentages by weight of nitrogen, phosphate and potash. On most labels the second and third are phosphate (P₂O₅) and potash (K₂O), not pure phosphorus and potassium.
How much nitrogen should a lawn get?
It depends on the grass, the soil, the time of year and local guidance, and some places limit how much you may apply. Use a soil test or your local advisory or extension service, and enter that figure as the target.
What is a common mistake when spreading fertilizer?
Spreading the label's nutrient rate as if it were the product rate. A target of 1 lb of nitrogen needs several pounds of product, so set the spreader to the product rate this calculator shows.
Can I convert kg per hectare to g per m²?
Yes: 1 kg per hectare is 0.1 g per m², and 1 kg per 100 m² is 10 g per m². Choose the unit under “Target rate unit” and the calculator converts it for you.
What does this fertilizer calculation leave out?
It assumes an even spread and does not account for slow-release nitrogen, soil levels or local rules on phosphorus near water. For a nutrient plan, ask a soil-testing lab or a local agronomist; the lawn watering calculator can help with watering it in.